Aluminum-base alloy



i Patented Mar. .20, 192 58.

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ROBERT s. Anem a, or cnnvnnann, OHIO, nssrenor; no .nnnnmun confirm or v AMERICA, A conrona'rion or PENNSYLVANIA.

alumna-1 :nnsn on,

1T0 Drawing.

The invention relates to aluminum alloys containing substantial amounts of silicon, and. has chiefly to do with such alloys which also contain iron.

5 The disadvantages of the presence of iron in more than minimal amounts. in aluminum allows containing substantial amounts of silicon have long been known. Not only does it impair the physical properties of these 511- loys, but it makes machinin ditficult, and tends to make the molten a1 0y sluggish so that permanent mold or pressure die casting is rendered difieult.

In some cases it is diflicult, if not impossible, to keep the iron content low. In the first place, iron may be present in aluminum by reason of its presence in the bauxite or other earths or materials from which aluminum is derived. Also, iron may be present '2 as a result of manufacturing condltions.

For example, aluminum silicon alloys have been found to be particularly adapted for making castings in iron molds and dies, and itis quite diflicult with present methods to v keep the iron content of the alloy. as low as is desired. Itis' necessary to keep the alumi-' num silicon alloy molten :Eor considerable periods of time -in the iron melting pots at tached to die casting apparatus, and ap so preciable quantities of iron are therefore continuously introduced in this way. Even in ordinary foundry work, the use of iron melting pots is common practice, and the introduction of iron from the melting pots '35 has always to be reckoned with.

silicon alloys containing 11 to about 12% silicon are normally compose oi primary aluminum dendrites with an aluminum silicon eutectic containing relatively 40 large plates and needles of silicon. Hypereutectic alloys containing more than about 12% silicon may contain, in addition to the eutectic, large crystals of excess silicon. In United States Patent No. 1,410,461 to J unius D. Edwards, Francis C. Frary and Harry -V.

Churchill, there is disclosed a process, known as a modification process, whereby the strength and ductility of these aluminum silicon alloys can be greatly increased and 60 the grain structure refined by the addition of alkali metals, sodium or potassium, to the alloys before casting. By this process the a ucauon'med time so, 1925; Serial an aorta normally coarse particles of eutectic, and particularly of the excess silicon, are very finely dispersed, with consequent increase in the strength and ductility of the alloy. If high ductility is desired in the resulting casting, it is very desirable to keep the iron content low, say below about 0.8%, because the iron seems to form a separate constituent in aluminum silicon alloys, taking the form of long, brittle plates or needles upon which said modification process has less eliminating effect or dispersing eflect than upon the" coarse particles of eutectic and of excess silicon. That the iron constituent in this form impairs strength and ductility has been shown by physical tests, as well as by observing the very coarse fractures which it produces when present in large amounts.

The objects of this invention are to provide an aluminum silicon alloy in which the deleterious efiects of the. resenceof iron are minimized'or neutralize and an alloy of the type stated which has improved ph sical properties, such as tensile strength an ductility.

The, invention is based on my discovery that the addition of certain metals to alumihum silicon alloys containing iron has a marked beneficial efiectupon the physical properties of the alloy, and has the efiect of minimizing or neutralizing the deleterious effect of iron particularly when the alloys contain relatively large amounts ofit, say above about 0.8%. The metals, one or more of which may be added to aluminum silicon alloys containing iron to produce these. advantages, are chromium, tungsten and molybdenum. The are metals of the sixth grou of the perio ic system having atomic weig is below 190,. and are .so defined throughout this specification and in the appended claims. Of these metals it is preferred to use chromium.

*In producin an alloy embodying the invention, an al oy of aluminum and silicon may be first prepared containin from about 3 to 15% silicon, which can be'( lone bystirring silicon into molten aluminum at a temperature of about 750 to 900 C. until dissolved. The necessary amount of one or more of the metals of the sixth group of the periodic system having atomic weights below 190 may then be introduced into the aluminum silicon alloy in the form of an aluminum alloy containing about-4% of the metal. While the alloy may, 'as stated, contain as little as 3% silicon, its preferredsilicon content is from 8 to As examples of the efi'ect of the addition of chromium to aluminum silicon alloys,

such-an alloy containing approximately 10% silicon and approximately'0.4% iron, when.

oured at 700 C. in sand molds to form alf-inch test bars, had atensile strength of 20,540 pounds per square inch and .an elonpounds per square inch, and its elongatio to 2% in two inches. When the iron conten was further increased to 2.42%, the tensile :strength was only-6,820 pounds per square gation of 4% in two inches. When the irorf content of this alloy was increased to 1.07%, its. tensile strength decreased --to 20,250

inclr-and'the elongation 0.5% in two inches.

-However, by the addition of chromium to these alloys, a substantial increase in strength was roduceih For example, in the .case of the ast-mentioned alloy which contained 10% silicon 'j and 2.42% iron, the tensile r strength was increased from 6,820 to 10,830

pounds 1' squareinch by the addition of 0.25% c 'romium. The addition of 0.44%

chromium further increased the tensile strength to 14,930 pounds per square; inch, andthe still further addition of chromium to 0.83% raised the. tensile strength to 18,175 pounds per square inch, the elongation remaining about 0.5%. in two inches, A 10% silicon alloy containing approximately 4% iron had a tensile strength of only- 3,025 poundsper square inch anda'n elongation 0t 0.5%, in two inches. The addition of 0.45%

chromium increased the tensile strength to 7,955. pounds per square inch; the addition of 0.86% chromium increased the tensile strength to 13,640 pounds per squareinch;

and 0.91% chromium increased the tensile strength to 15,950 pounds per square inch.

The foregoing examples of the advantage of'the addltion of chromium to aluminum silicon alloys are all of alloys cast in sand molds. As showing the advantage of the-ad- "dition of chromium to aluminum silicon alloy's cast in iron molds, such an alloy contaming 13.65% silicon and 1.25% iron, when I i i cast in an iron mold in the form of half-inch test bars showed an average tensile; strength of 28,625 pounds per square inch and an average elongation of 2.3% in two inches for f fourtest bars. .By adding 0.4% chromium to this alloy,'the averagetensile strength of four test. bars was increased to 30,915 pounds invention-in die cast'aluminum silicon al-h er square inch, and the elongation to 2.5% mtwo inches. 1 r As an example of the advantage of the loys, such an alloy containing approximate-i 1;; 13% silicon and about 2.5% iron showed,

] when die-cast in the formbf test bars 0.252 inch in diameter, an average tensile strength of 30,246 pounds per square inch and an elongation of 0.8% in two inches. By adding 0.125%, chromium to this alloy and forming similar die cast test'bars from it,

it was found tohave an average tensile strength of 33,504 pounds per square inch.

and an elongation of 1.9% in two inches.

That the advantages of the invention are realized in alloys containing smaller amounts of silicon than those given in the specific examples to which reference has thus far been-made, is shown by the following: An aluminum alloy containing approximately 5% silicon and 3% iron had an average ch and an elongation of 1.9% in two inches when cast in sand in the form of half-inch test bars. 'The addition of 0.125% chromium nsile strength of 16,015 pounds per square raised the average tensile strength to 17,150

pounds per square inch and the elongation to 3.1% in two inches. Further increasing the chromium ,to 0.25% increased the tensile strength to 17,730 pounds per square inch and the elongation to 3.8% in two inches.

'It is well known that the production of homogeneous alloys of aluminum with high melting metals, such as chromium, tungsten, iron, etc.,iisoften attended with some'adifficulty. It is known that this; can be'overcome in art by casting, remelting and 'a ain casting t e alloys. By usin this proce are,

the physical properties of t e aluminum al- 'loys containing 5% silicon to which reference was last made, were still further increased. The alloy with 0.125% chromium, when subjected to this procedure, possessed after remelting an average tensile strength of 18,140 pounds persquare inch, and the,

alloy with 0.25% chromium possessed a tensile strength of 18,860 pounds per square inch.

As an example of the advantage of incorporating tungsteninan aluminum silicon alloy, such an all'o containing. about 10% silicon and about 3 0 iron whencast in sand an average tensile strength of 6,670 pounds per square inch. After adding 1% oftun sten to thisalloy, tensile strengths as hig corporating molybdenum in aluminum silicon alloys, the addition of about 1% molybabout 10% silicon and about 4%. iron, in-; creased the tensile strength ofsandcast test bars from 3,850 to 13,625 pounds per square inch. The additionfofsmaller amounts of molybdenuin produced tensile strengths between these.-

Reference has heretofore beert niade to the fact that the medi'fication' process vloj alurn'ipounds per square inch were ob-.

fldenum. to an aluminum; alloy containing- 'num silicon alloys does not eliminate the deleterious eflects of the presence of iron in such alloys. As an example of the advantage of this invention in alloys which have been subjected to the modification process, an aluminum silicon alloy containing about 10% silicon and about 3% iron was cast in sand molds in the form of half-inch test bars, and showed an average tensile strength of 6,670 pounds per square inch. The same alloy was treated at 927 C. with 2% of the refining flux described in United States Pat ent No'. 1,387,900 to A. Pacz, and after being cast in sand molds in the form of half-inch pounds per square inch.

A m1croscop1c examination of the various, alloys described in the foregoing examples, and other iron-containing aluminum silicon alloys, shows that the addition to aluminum silicon alloys of a metal of the sixth group of the periodic system having an atomic weight below about 190 produces a marked effect. 'With chromium, for example, a new constituent, which usually takes the form of equiaxed particles appears, and the coarse,

needle-like structure of the iron particles al-' most entirely disappears. The iron content appears to" be largel associated with the constituent of the ad ed metal, but some of it maybe present in finely dispersed form.

The coarse fracture usually associated with high iron in aluminum .silicon alloys becomes finer and finer with increasing additions of the added metal.

Referring particularly to the addition of chromium, its effect in eliminating the brittle, needle-like iron crystals ordinarily pres- I ent in aluminum silicon alloys is most some conditions in alloys containing less marked in the alloys of high silicon content, say from 10 to 15%, which also contain relatively large amounts of iron. However, the use of chromium is advantageous under amounts of silicon, say as low as about 3%. The preferred chromium content varies somewhat with the content of iron and silicon. fWhen silicon is present within the 'limit stated, from about 3 to 15%, and the iron does not exceed about 3%, the preferred chromium content is from about 0.25 to 2%, depending upon the properties desired. Under some circumstances, chromium contents as low as 0.05% may be beneficial.

When the iron content does not exceed about 1.5% the use ofthe modification processes to which reference has been made produces a ibeneficial efiect upon the physical. properties largely by refining the grain and increasing the dispersion of the silicon. Chromium may also be added to aluminum silicon alloys with the advantages prev ously explained, although when the iron content exceeds about 1.5% .the simultaneous use of the modification process and the addition of chromium has little, if any, advantage, and when the iron content exceeds about 1.5%,

I usually prefer to add chromium. to the alloy without subjecting it to the modification process.

In addition to refining the grain and elimie nating the needle form of the iron constituent of an aluminum silicon alloy, the addition of a metal of the sixth group of the periodic system having an atomic weight below 190, and particularly the addition'of chromium, apparently increases the fluidity of the alloy. This is of marked advantage in the use of the alloy in die casting and in iron molds where a very fluid alloy is necessary in order to completely fill the molds before solidification takes place. By eliminating, or reducing the brittle, needle-like form of the iron constituent, these metals,

and particularly chromium, also act to make the resulting casting more easily machinable, which is an additional great advantage.

According to the provisions of the atent statutes, I have explained the princip e and operation of my invention, and have given numerous specific examples of various embodiments of it. However, I desireto have it understood that, within the scope. of the appended claims, the invention may be racticed otherwise than as herein speci cally described. I claim as my invention: I i

.1. An aluminum basealloy c ntaining from about 3 to 15 per cent sili iton with more than about 0.8 per cent iron,'and more than about 0.05 per cent of a metal of the sixth group of theperiodic system having an atomic weight less than 190.

,2. An alummum base alloy containing from about 3 to 15 per cent silicomwith more than about-0.8 per cent iron, and also containing from 0.05 per cent to about 2.0

per cent of a metal of the sixth group of the periodic system having an atomic .weight less than" 190.

oup of i substantially reduce I like iron constituent. a 5. An aluminum base alloy containing from about 3' to 15 per cent silicon with the plate and needle- 6 more thanabout 0.8 per cent iron, and also containing more than about 0.05 "per cent of chromium.

6. An aluminum base alloy containing from about 3 to 15 per cent silicon with more than about 0.8 per cent iron, and also ROBERT is. ARCHER. 

